Certain types of fluid flow control devices include a regulator which limits the maximum or minimum (extreme) level of some parameter of the fluid flow such as pressure or flow rate. A particular type of such a flow control device which is of interest here is a gas valve which has integral with it a regulator which maintains a preset minimum outlet pressure. However, the discussion which follows is applicable to applications for limiting maximum pressure, to fluids different from the types mentioned (including liquids), and to flow parameters other than pressure. It is preferable for certain applications that the installer be able to adjust the setpoint of the regulator so that the extreme parameter value modulated by the regulator conforms to the requirements of the system in which it is installed.
In the type of gas valve mentioned in the above paragraph, the setpoint pressure of the pressure regulator is adjustable by the setting of an adjustment screw within a cylindrical bore on the body of the valve. The interior wall of the cylindrical bore is threaded to match the adjustment screw's threads. By rotating the adjustment screw in one direction to advance it further into the bore or in the other to withdraw it from within the bore, the minimum pressure setting of the regulator may be respectively reduced or increased.
It is important when dealing with certain types of fluids such as natural gas or propane (liquid petroleum) to assure at all times that the pressure of the gas at the outlet port of the control device is above an absolute minimum value regardless of the requirements of the installation. If this minimum pressure is not always present, it is possible that unsafe operation of the burner receiving the gas flow will result. For example, it is necessary for safe operation of a natural gas burner that the outlet pressure never fall below approximately 0.03 psig (200 Pascals). For propane, the outlet pressure should never fall below 0.05 psig (350 Pascals).
It is necessary that the adjustment screw be accessible in the field so that the installer can set the minimum pressure to an optimum value higher than the absolute minimum value for the fluid to conform the value to the requirements of the system for economical and efficient operation. Pressures lower than this optimal value (but above the absolute minimum value) are safe but just not as desirable. By providing the installer with this adjustment option, it is possible to stock fewer valves for a given capacity for example, thereby lowering overall cost for providing the valves of various types.
On occasion, it has been found that the pressure adjustment screw of such valves will be positioned outwardly past the extreme withdrawn safety setting, causing the outlet pressure to be too low for safe operation. It is therefore preferable that there be some mean for preventing incorrect adjusting of the adjustment screw which results in excessively low outlet pressure.
One possible means for accomplishing this is to mold a feature such as a shoulder in the interior of the cylindrical wall to prevent withdrawal of the adjustment screw past the extreme withdrawn safety setting. Unfortunately, the tolerances of the parts from which these pressure regulators are made, and the assembly process, are not so precise and repeatable as to allow such a fixed position feature to reliably and inexpensively reproduce the low pressure set point in a production setting. Other approaches also seem to have serious cost or reliability penalties. Accordingly, there is a substantial need for a means for limiting the setting of the adjustment screw to prevent it from being withdrawn past the extreme withdrawn safety setting establishing the minimum outlet pressure of the valve.